The present invention relates to an optical information recording medium suitable for recording information therein and reproducing the same therefrom by application of laser beams, in particular, suitable for application of semiconductor laser beams.
Recently an information recording medium including an organic recording layer comprising an organic dye is attracting more attention than an information recording medium including a metallic recording layer comprising, for example, Te. This is because the organic recording layer is advantageous over the metallic recording layer in that (1) the melting point, decomposition temperature and thermal conductivity of organic dyes are much lower than those of metals such as Te, so that a recording medium including the organic recording layer can attain higher recording performance and higher recording density than the metallic recording layer, (2) the organic layer can be formed by coating and therefore is more suitable for mass production than the metallic coating layer and lower priced than the metallic recording layer, and (3) when the organic dye layer is formed on a substrate made of a plastic material, since the recording layer is similar in properties to those of the substrate, in particular, in the physical properties such as thermal expansion and water penetration, the occurrence of cracks in the recording layer and warping of the substrate can be reduced as compared with the case where the metallic recording layer is employed.
However, when an organic recording layer containing an organic dye is formed on a plastic substrate by coating, a solvent such as acetone and dichloroethane, in which the plastic substrate is highly soluble, is employed in a recording layer coating liquid. The result is that it becomes a problem that even the plastic substrate is dissolved in such solvent during coating.
Once the plastic substrate is dissolved in the solvent during the coating of the organic dye layer, even if the extent is extremely small, the reflection ratio of the organic layer cannot be increased any further. As a result, the above-mentioned advantages of the organic layer cannot be attained any longer. In order to avoid this problem, it has been proposed to use a solvent which does not dissolve the substrate. However, such a solvent has the drawbacks that the preservability and recording characteristics of the recording medium are impaired and pin holes are formed in the recording layer.
An inorganic recording layer comprising an inorganic material also has the drawbacks that the recording sensitivity is low because of the high thermal conductivity of the inorganic material contained in the recording layer and cracks are apt to be formed in the recording layer, because of much difference in the thermal expansion ratio and the expansion by water absorption between the inorganic recording layer and the organic substrate, by which recording error may be caused.